Membrane Grating for Beam Steering Device and Method of Fabricating Same

ABSTRACT

A method of fabricating a membrane structure for a diffractive phased array assembly is provided. The method includes the steps of providing a wafer having a body and at least a membrane layer and a backside layer disposed on opposite faces of the body, forming a grating pattern on a surface of the membrane layer, and forming a window through the wafer to expose a back surface of the membrane, thereby allowing light to pass through the membrane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of priority of U.S. ProvisionalPatent Application No. 60/922,128, filed Apr. 6, 2007, which applicationis hereby incorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

The subject invention is directed to a diffractive phased array assemblyused as a beam steering device in an optical system, and to a method offabricating and constructing such an assembly.

2. Description of Related Art

Diffractive phased array assemblies are used as beam steering devices inmany optical systems such as bar code scanners, laser machiningapplications and laser printers, as discussed in U.S. Pat. No. 6,480,334to Farn, the disclosure of which is herein incorporated by reference inits entirety.

A known diffractive phased array design used as a beam steering devicein LIDAR (Laser Imaging Detection and Ranging) applications is formedfrom 2-inch square silicon plates. The surfaces of these plates arepatterned with complimentary microscopic grating features that are usedto steer a monochromatic laser beam of a prescribed wavelength when thearray plates are placed in intimate contact and one plate istransversely displaced with respect to the other.

In the current state of the art design, which is shown for example inFIG. 1, the plate thickness d is about 1-2 mm, the plate spacing g isapproximately 1 μm and the depth of the grating features isapproximately 0.5 μm. This design yields a theoretical 0^(th)-orderoptical throughput that is close to unity and remains above 50% forangles θ as large as +/−20°. However, the throughput falls offdramatically for gap sizes that are greater than 1 μm.

The sensitivity of the optical throughput to the localized plate spacingsignificantly limits the usefulness of this state of the art design.Moreover, substrate imperfections such as warp and wedgenon-uniformities can be significantly larger than 1 μm across the faceof the plate, even for the highest quality material available.Additional warping can occur when the plates are mounted to atranslation mechanism used to vary the output beam angle.

Applicant recognizes that the manufacturability of the state of the artassembly can be markedly improved by replacing one or both plates withmembrane structures, as shown for example in FIG. 2, which illustratesone device in accordance with the invention. In such an instance, thesubstrate 210 is removed from the grating area 220, leaving only amembrane 225 of thickness d′ which could be on the order of the platespacing, as explained in more detail below.

SUMMARY OF THE INVENTION

The subject invention is directed to a new and useful diffractive phasedarray assembly for use as a beam steering device in an optical system.Such beam steering devices can be utilized in a beam sending capacity,such as in close proximity to a laser intended to scan a particularfield of regard. Alternatively or additionally, such beam steeringdevices can be utilized in conjunction with sensors that are adapted andconfigured to scan a field of regard.

In accordance with one aspect of the invention, a method of fabricatinga membrane structure for a diffractive phased array assembly isprovided. The method includes the steps of providing a wafer having abody and at least a membrane layer and a backside layer disposed onopposite faces of the body, forming a grating pattern on a surface ofthe membrane layer, and forming a window through the wafer to expose aback surface of the membrane, thereby allowing light to pass through themembrane.

In accordance with any embodiment of the invention, the step of forminga window through the backside layer and wafer body can be performedthrough a photolithographic technique, and can includes the steps ofapplying a photoresist layer on the face of the backside layer, exposingthe photoresist in a pattern to define the window, removing thephotoresist in regions where etching is to occur, and etching through atleast the backside layer to partially form the window. The photoresistlayer can be any thickness necessary, but in a preferred embodiment isabout 7 μm thick.

In accordance with any embodiment of the invention, the wafer body canfirst be etched to a predetermined depth with a first etching technique,and subsequently etched with a second etching technique to complete theetch of the wafer body. The first etching technique can be deep reactiveion etching, and the second etching technique can be a chemical etch,such as one with tetramethylammonium hydroxide. The predetermined etchdepth can be about 100 μm, for example, but depend on the thickness ofthe wafer. The thickness of the membrane layer can be as desired, forexample between about 10 μm and 75 μm.

If so desired, in connection with any embodiment of the invention, thewafer can further include an etch-stop layer underlying the membranelayer. Accordingly, the method can further include the steps of etchingthrough the wafer to partially form the window, and etching through theetch-stop layer to the membrane layer to fully form the window.

Fabrication of a membrane structure can further include the step ofmounting the membrane structure in a protective fixture prior to thestep of etching of the etch stop layer, in order to protect the membranelayer. The etch stop layer, in the designated area, is removed with asuitable substance, for example with a buffered oxide etch.

The materials of the wafer body, backside layer, the etch stop layer,and the membrane layer can be selected so that when a window is formedthrough the wafer up to the membrane layer, the materials thereinexhibit internal or hoop stresses sufficient to provide tension to themembrane layer, to maintain the membrane layer substantially flat.

The wafer body can be formed of any suitable material, such as silicon,the backside layer likewise can be formed of any suitable material, suchas silicon nitride, the etch stop layer can be formed of any suitablematerial, such as silicon dioxide, and the membrane layer can be formedof any suitable material, such as silicon.

In accordance with any embodiment of the invention, a method ofmanufacture can include the step of confirming that the flatness of themembrane structure is within allowable limits.

Methods in accordance with the invention can further include the step ofbaking the membrane structure for a length of time at a temperature andhumidity sufficient to promote sufficient oxide growth on the membrane.This step can be performed in an aqueous medium, such as in a beaker ofwater, for example.

In accordance with another aspect of the invention, a diffractive phasedarray assembly includes first and second membrane structures. Eachmembrane structure has a membrane layer with a patterned grating definedthereon. The membrane layers of the two membrane structures are inoperative contact with one another so as to form a gap therebetween.

An index matching fluid can be provided within the gap formed betweenthe patterned surfaces of the first and second membrane structures. Theindex matching fluid can be one that has an ultra-low viscosity.

The first and second membrane structures can be adapted for relativetranslational movement, substantially parallel to the membrane surface.

Each of the first and second membrane structures can be provided withinternal stresses to apply tension to the membrane layer to maintain themembrane layer flat.

In accordance with another aspect of the invention, a diffractive phasedarray assembly includes a plate structure having a patterned gratingdefined thereon and a membrane structure having a patterned gratingdefined thereon, wherein the patterned grating of the plate and thepatterned grating of the membrane are in operative contact with oneanother so as to form a gap therebetween. An index matching fluid can beprovided within the gap formed between the patterned surfaces of theplate structure and membrane structure.

The plate structure and membrane structure can be adapted for relativetranslational movement, substantially parallel to the respectivegratings.

As mentioned, it is envisioned that a gap between adjacent gratings canbe filled with a low viscosity, refractive index matching fluid, whichfunctions to pull the two membrane layers together through surfacetension, so as to maintain a uniform gap. The fluid should havemechanical and thermal properties that are suitable for a variety ofoperating environments, including high and low temperature environments.Further, the presence of the fluid may additionally serve to providedamping to reduce any undesirable vibrations. However, it should beunderstood that devices in accordance with the invention can be providedwithout such a fluid. Alternatively still, devices in accordance withthe invention can be embodied such that they are fully enclosed in afluid filled space, as will be described in more detail below.

Alternative fabrication methods are also envisioned. For example, atimed etching step can be employed to alleviate the need for anetch-stop layer below the membrane layer. It is also envisioned that themembrane can be fabricated by a dry etching process or by a wet etchingprocess.

These and other aspects of the diffractive phased array assembly andmethod of fabrication will become more readily apparent from thefollowing detailed description of the preferred embodiments taken inconjunction with the drawings briefly described below.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject invention pertainswill readily understand how to make and use the subject inventionwithout undue experimentation, preferred embodiments thereof will bedescribed in detail hereinbelow with reference to certain figures,wherein:

FIG. 1 illustrates a state of the art assembly wherein patterned platesof a thickness d are separated by a gap g, whereby the relativetranslation of the plates shifts the direction of the output beam λ byan angle θ;

FIG. 2 illustrates a pair of patterned membranes of thickness d′separated by a gap g;

FIG. 3 a illustrates the state of the art plate design in which uniformplate spacing cannot be maintained as a result of imperfect substrates;

FIG. 3 b illustrates how strain is used to pull the thin membranegratings flat, allowing uniform spacing therebetween, and wherein theuniform spacing is ensured by placing an appropriate index matchingfluid in the gap between the membranes;

FIGS. 4 a through 4 e illustrate the steps for fabricating a membranestructure in accordance with a preferred embodiment of the of thesubject invention.

FIG. 4 f is an isometric view illustrating a finished membrane structuremanufactured in accordance with the invention; and

FIG. 5 is a cross-sectional view of an alternate aspect of theinvention, illustrating a conventional plate-like structure used inconjunction with a membrane structure constructed in accordance with thepresent invention, with an optional housing provided therewith.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The benefits of the subject invention are shown in FIGS. 3 a-3 b, whichcompare a conventional diffractive phased array plate assembly 103 to anassembly 300 having membrane structures formed by a method in accordancewith the subject invention. FIG. 3 a illustrates a realistic case wherethe substrates 110 a, 110 b are slightly warped, demonstrating thedifficulty in maintaining relatively small uniform plate spacing. Forexample, the plates could be in intimate contact at one location 2,while the gap between the plates could be tens of microns or larger atanother location 1.

FIG. 3 b illustrates how the two patterned gratings can be brought intooperative uniform contact using a membrane grating approach. If the netstrain of the combined structure is such that the thin film membranelayer 225 is pulled tightly across a window 209 a, 209 b formed in thesubstrate 210 a, 210 b, it will no longer exhibit the curvature of thesubstrate 210 a, 210 b. While this serves to reduce or eliminate theeffects of substrate curvature, depending on the precise conditionsdevices in accordance with the invention experience, there still may bechallenges in maintaining a small spacing (e.g., of about 1 μm) betweentwo membrane gratings 220 a, 220 b. For example, the spacing may beaffected by thermal or mechanical perturbations. Applicant hasdetermined that one manner suitable for overcoming such problems isplacing a refractive index matching fluid 380, which can be an ultra-lowviscosity fluid, between the two membrane gratings 220 a, 220 b, asshown for example in FIG. 3 b. Such fluids may include, for examplewater, mineral oil, or other index matching fluids typically used inoptical applications. If a hydrophilic fluid is specified, the fluidwill wet the membrane gratings 220 a, 220 b and surface tension willpull the gratings 220 a, 220 b together and maintain a uniform spacingtherebetween.

As illustrated in FIG. 5, it is envisioned and well within the scope ofthe subject disclosure that a diffractive phased array assembly canalternatively be constructed using one conventional plate structure, asillustrated in FIG. 3A, for example, and one membrane structuremanufactured in accordance with the present invention, as illustrated inFIG. 3B, for example. In accordance with this embodiment, such plate andmembrane structures each are provided with a patterned grating definedthereon. Although not required, in such an instance the membranestructure can be arranged so that the membrane grating effectivelyblankets the plate structure, allowing the gratings to be in mutualoperative contact with one another, with a defined spacing or gaptherebetween. Alternatively, the gratings can simply be brought intoclose contact with one another Although not required in accordance withthe invention, Applicant conceives that the membrane grating canadvantageously be pulled closer to the plate grating by providing afluid, such as an ultra-low viscosity fluid therebetween.

Referring now to FIGS. 4 a through 4 e, there are illustrated examplesteps of fabricating a membrane grating in accordance with one aspect ofthe invention. In accordance with this aspect, a first step (FIG. 4 a)includes providing a starting material that includes a Si (Silicon)layer 430 having an upper Si membrane layer 225, with an interveningetch-stop layer 450. The etch stop layer 450 can be, for example, SiO₂(Silicon dioxide). A backside layer 440, which can be Si₃N₄ (SiliconNitride), for example is also provided for structural rigidity and toprovide, if desired, internal tensile hoop stresses, resulting in abending moment (indicated by “M” in FIG. 4 e) to the assembly to assistproviding tension (indicated by “T” in FIG. 4 e) to the membrane 225 andgrating formed thereon The silicon nitride layer 440 also serves as amore robust mask layer during the etch of the silicon layer 430.Photoresist is usually adequate for a plasma or reactive ion etch.However, in accordance with the invention, a combination of dry and wetetches can be used for deep silicon etching. The photoresist layer 490,may be less effective during a wet etch portion. Accordingly, thenitride layer can serve to protect against undercutting of thephotoresist 490, which could otherwise compromise the silicon etch.Thereafter, as shown in FIG. 4 b, a grating pattern is defined on themembrane layer 225, which can be accomplished in any suitable fashion.The membrane can be defined, by way of general binary optics technology,as set forth in U.S. Pat. No. 6,480,334 to Farn.

As shown in FIG. 4 b, a photoresist layer 490 is applied to the backside of the backside layer 440. The photoresist can be applied prior toor following the above-mentioned step of defining a grating in themembrane layer 225, provided that the process of forming such gratingdoes not damage the photoresist layer. The photoresist layer can be ofany thickness required, which is about 7.0 μm, in one embodiment. Eithera positive or negative photoresist can be used, such as AZ9260®,manufactured by MicroChemicals GmbH of Ulm, Germany, for example.

Then, the photoresist is exposed. Applicant has used a Quintel 1200,manufactured by Neutronix-Quintel of Morgan Hill, Calif., USA, thatallows exposure of the photoresist while allowing for manual alignmentof the wafer, which is more accurate than auto-alignment systems ofother similar devices. The photoresist layer 490, in areas where etchingis desired is then removed.

Subsequently, as shown in FIG. 4 c, a window 209 is etched in thebackside layer 440 to expose the Si layer 430. In accordance with theinvention, this step can be performed by any suitable technique. In apreferred embodiment, this step is performed with DRIE (Deep ReactiveIon Etching). DRIE is a highly anisotropic etch process suitable forcreating deep, steep-sided holes and trenches in wafers, with highaspect ratios.

Etching of the window 209 through the Si layer 430 is continued, whichetching stops at the etch stop layer 450, as shown in FIG. 4 d. Inaccordance with the invention, this step can be performed by anysuitable technique. In a preferred embodiment, this step is performedwith tetramethylammonium hydroxide (TMAH). It should be noted that thedrawings should not be relied upon for illustrating precise relativemeasurements. For example, the slope of the sidewalls of the backsidewindow 209 formed in the silicon layer 430 may be more or less steepthan illustrated in FIGS. 4 d and 4 e. Similarly, the precisethicknesses of layers cannot be judged by referring solely to thedrawings.

Optionally, subsequently the wafer 400 d can be mounted in a protectivefixture to prevent damage to the membrane 225 during removal of the etchstop layer 450 in the region of the backside window 209.

The etch stop layer 450 in the region of the backside window 209 is thenremoved by a suitable compound. The membrane 225, which is formed froman upper membrane layer 225 is fully defined by etching the etch stoplayer 450 in the backside window 209 d, to yield a backside window 209configured as shown in FIG. 4 e. The etch-stop SiO₂ layer allows for aconsistent and accurate membrane thickness of the membrane 225, and thusfor the grating 220. Etching of the etch stop layer 450, which in thepresent embodiment is silicon dioxide, may be accomplished by using asuitable etchant, preferably a BOE (buffered oxide etch), due to theselective nature of such an etch. Alternatively, the silicon dioxidelayer can be etched with a non-buffered hydrofluoric acid (HF) solution,for example.

Subsequently, the flatness of the membrane 225 can be measured. A 2-dprofiler, such as a Cyberscan Vantage, manufactured by CyberTechnologies, GmbH of Ingolstadt, Germany can be used for this purpose.In accordance with the invention, the flatness of the whole assembly 400e, or solely that of the membrane 225 in the region of the window 209 ecan be measured. In accordance with the invention, it is preferred thatthe membrane flatness be verified both after performing the last etchstep, and after mounting the assembly 400 e to a holder, if any. Thatis, the assembly 400 e can be mounted to a holding structure forincorporation into a larger optical system. It is preferred thatflatness is confirmed at that point as well, should any non-uniformstresses be applied to the assembly 400 e, and hence to the membrane225. In accordance with the invention, flatness measurements can betaken in one direction and then again perpendicular to that direction(e.g., along a defined “x-axis” and then a perpendicular “y-axis”).

Subsequently, the assembly 400 e can be baked to promote sufficientoxide growth, to allow the grating 200 to wet. This step can be repeatedas necessary to achieve the desired results. In accordance with theinvention, this step can occur at about 95° C. for about 2.0 hours.Typically, semiconductor surfaces are “passivated” or stabilized.Although silicon naturally oxidizes by forming a very thin layerthereon, such a layer may not be sufficiently thick to render thesurface thereof sufficiently hydrophobic for use in conjunction with anintervening fluid, in accordance with the invention.

Forming a window 209 in the membrane grating 400 allows internalstresses developed within the composite structure to deform the annularregion surrounding the window 209. The internal compressive hoop stressof the etch stop layer 450, serves to pull the membrane 225 flat, andensures that the edges of the silicon layer 430 do not interfere withintimate positioning of adjacent membranes of companion gratingassemblies 400. Preferably, the backside layer 440 is essentiallystress-free or exhibits slight internal tensile hoop stress, so as toprevent countering the strain effects of the etch stop layer 450. Thethickness of the membrane 225, in accordance with one aspect, should beas large as possible, limited by the ability of the internal stresseswithin the etch stop layer 450 to pull the membrane 225 flat. Inaccordance with the invention, the membrane 225 is between 3.0 and 100.0μm (microns). In preferred embodiments, the thickness of the membranelayer 225 is one of about 10.0 μm, about 50 μm and about 75 μm.Naturally, the precise thickness can be selected as desired, and beanywhere in the range between about 0.5 μm and 3000.0 μm (3.0 mm) inthickness, at any 1.0 μm increment or portion thereof therebetween.Membranes in accordance with the invention can be even larger, if sodesired. Naturally, depending on the precise thickness of the membrane,the robustness of remaining structure should be increased so as tomaintain sufficient tension in the membrane to maintain the membraneflat.

In accordance with the invention, the assembly 400 can have any desiredshape or size practical. Although there are no concrete limitations onsize of the assemblies 400 in accordance with the invention, the size ofthe window opening 209 will depend on the optics of the associatedapplication. For example, to steer an outgoing beam the window 209 canbe on the order of the beam size, for example as small as between 1 mmand 10 mm. If a return signal must also be captured, the windowdimension will be set by the optical design of the collection optics.Typically in such an instance, larger sizes would be preferred, butprocessing difficulties would limit that size. In accordance with theinvention, a 2 inch (5 cm) diameter window 209 can be provided.Regarding membrane thickness, a thicker membrane 225 is preferred.However, a thicker membrane 225 can cause increased difficulties incompensating for its strain, as discussed herein.

Additionally, the optical distance (or “gap”) between the two gratings220 a, 220 b should be minimized. It is conceived by Applicant that anassembly including two gratings can be provided where the gratings areprovided on opposite sides of the membrane 225, from the illustratedarrangement. This would provide a larger spacing between the membranesthemselves, with a fluid (gas or liquid) disposed therebetween. Such anarrangement may ease fabrication but would result in gratings evenfarther apart than illustrated herein, which would affect steeringefficiency. Similarly, but likely to suffer even greater steeringefficiency losses, each assembly 400 of the pair utilized (as in FIG. 2)can be reversed, having the substrate 210 portions adjacent to oneanother with the membranes 220 arranged on opposite sides thereof.

With respect to the layer thicknesses, the membrane layers arepreferably on a micron-scale e.g., between 5 μm and 1000 μm, forexample. With a larger area of the membrane 225 exposed by the window209, a thicker membrane layer 225 is typically required to ensuremechanical stability. If the membrane layer 225 is transparent to thelight source being used, such as a particular frequency of laser,absorptive losses are not an issue. In such an instance, mechanicalstability and strain balancing drive the minimum and maximum thicknessfor the membrane 225. The thickness of the etch stop layer 450 ispreferably minimized and is typically limited by the selectivity of theetching process. Accordingly, the thickness of the etch stop layer 450is between about 1000 and 5000 angstroms. The thickness of the backsidelayer 440 is then designed to compensate for the etch stop 450 strainand ensure the membrane 225 exhibits sufficient strain. As can beappreciated, precise dimensions are dependent on a number of factors,that are tied together and depend significantly on the specificmaterials used.

As illustrated, the starting material 400 a, which includes a siliconsubstrate, etch stop layers and the like, has a substantially circularcross section—that is, the material is substantially cylindrical. Inaccordance with one aspect of the invention, the starting material ispreferably about 4 inches (10 cm) in diameter. The finished assembly 400e, in accordance with this aspect, has a membrane 225 spanning a window209 e having a diameter of between 1 inch and 2 inches (2 cm and 5 cm)with a substantially circular cross-sections. However, such particularsize and shape are not required in accordance with the invention.

In accordance with the invention, manufacturing methods of devices setforth herein can include the steps of confirming the flatness of amembrane grating 220, and baking the membrane gratings to promote nativeoxide growth thereon.

FIG. 4 f is an isometric view illustrating a finished membrane structure400 e of FIG. 4 e, manufactured in accordance with the invention.

Processing of a silicon wafer to arrive at the starting material 400 ashown in FIG. 4 a can include any of a variety of suitable steps. Forexample, the etch stop layer 450, if silicon dioxide, can be formed bytreating the surface of the silicon wafer 430. The etch stop layer 450can be deposited, by way of vapor deposition, for example or anothertechnique. Alternatively, the etch stop layer 450 can be formed betweenthe main silicon layer 430 and the membrane layer 225 by way of ionimplantation such as SIMOX (Separation by Implantation of Oxygen), waferbonding techniques, or seed techniques. Such techniques can be utilizedto manufacture the starting material in accordance with the invention,as will be recognized by those skilled in the art.

The backside layer 440 can be applied to the silicon wafer 430 in any ofthe foregoing manners, such as by way of a wafer bonding technique.Alternatively, and in the case of a backside layer 440 formed of siliconnitride on a silicon substrate, the backside layer 440 can be formed byreacting the silicon wafer 430 with nitrogen gas at high temperature.Alternatively still, chemical vapor deposition or plasma-enhancedchemical vapor deposition techniques can be utilized in accordance withthe invention.

Similarly, devices in accordance with the invention can be manufacturedby using wafer bonding techniques and/or deposition techniques, withoutetching, or in combination with etching steps, as long as the resultingmembrane grating is maintained flat when completed. Applicant haveconceived, as set forth herein, that an advantageous manner ofmaintaining a membrane grating flat is to form it in connection with asupporting structure that applies tension to the membrane. In theaforementioned embodiments, this is due to internal stresses developedwithin the supporting structure.

FIG. 5 is a cross-sectional view of a further aspect of the invention,illustrating a membrane grating assembly 500, including a conventionalplate-like structure 110, as those illustrated in FIG. 1, used inconjunction with a membrane structure assembly 400 constructed inaccordance with the present invention, as those illustrated in FIGS. 2,3 b and 4 a-4 f, for example. As illustrated, a membrane gratingassembly 500 includes two gratings 120, 220, respectively disposed onthe plate structure 110 and membrane structure assembly 400. As with anyembodiment set forth herein, the assembly 500 is adapted for relativetranslational movement to allow the assembly to steer a beam of lightpassing therethrough.

As set forth above, a fluid can be supplied to fill the gap 570 betweenthe two gratings 120, 220. Alternatively, a fluid can fully surround theassembly 500, including in-between the gratings 120, 220, as indicatedby housing 599. Such an arrangement may be useful in situations whereadditional protection from damage is desired and/or the damping effectsof a surrounding fluid would be advantageous. Nevertheless, the portionof fluid that is provided in the gap 570 serves to maintain a consistentspacing between the plates and hence the accuracy of the device as awhole.

It is preferred that the material selection, including substrates formembrane and plate grating manufacture have a relatively high refractiveindex in comparison with the medium in which the device will operate.Although this medium is typically air, depending on the preciseimplementation, that may not be the case. Similarly, the fluids disposedbetween and/or around the gratings would preferably have a relativelylow index of refraction in comparison with the grating materials. Such acombination of high index materials results in a device that is capableof steering a beam of light across a broader field. Additionally, thematerials selected should have very good transparency for thewavelengths of light with which they are to be used.

Once the device is assembled, it can be calibrated, based on therefractive indices of the particular materials used, and can thereafterbe successfully deployed as a beam steering device.

While the diffractive phased array assemblies and methods of fabricationof the subject invention have been shown and described with reference topreferred embodiments, those skilled in the art will readily appreciatethat various changes and/or modifications may be made thereto withoutdeparting from the spirit and scope of the subject invention as definedby the appended claims. Particularly, it is to be understood that theprecise methods of manufacture can vary from those set forth explicitlyherein, perhaps utilizing different materials and/or processingtechniques and/or different processing chemicals, as will be appreciatedby one skilled in the art.

Further, it is to be understood that elements of devices or steps ofmethods described in connection with a particular embodiment may beadvantageously applied to other embodiments of the invention, and arenot limited to specific embodiments, unless explicitly described assuch.

1. A method of fabricating a membrane structure for a diffractive phasedarray assembly, comprising the steps of: a) providing a wafer having abody and at least a membrane layer and a backside layer disposed onopposite faces of the body; b) forming a grating pattern on a surface ofthe membrane layer; and c) forming a window through the wafer to exposea back surface of the membrane, thereby allowing light to pass throughthe membrane.
 2. A method of fabricating a membrane structure accordingto claim 1, wherein the step of forming a window through the backsidelayer and wafer body is performed through a photolithographic technique,and includes: a) applying a photoresist layer on the face of thebackside layer; b) exposing the photoresist in a pattern to define thewindow; c) removing the photoresist in regions where etching is tooccur; d) etching through at least the backside layer to partially formthe window.
 3. A method of fabricating a membrane structure according toclaim 2, wherein the photoresist layer has a thickness of about 7 μm. 4.A method of fabricating a membrane structure according to claim 2,wherein the wafer body is first etched to a predetermined depth with afirst etching technique, and subsequently etched with a second etchingtechnique to complete the etch of the wafer body.
 5. A method offabricating a membrane structure according to claim 4, wherein the firstetching technique is deep reactive ion etching, and the second etchingtechnique is a chemical etch using tetramethylammonium hydroxide.
 6. Amethod of fabricating a membrane structure according to claim 4, whereinthe predetermined depth is about 100 μm.
 7. A method of fabricating amembrane structure according to claim 2, wherein the thickness of themembrane layer is between about 10 and 75 μm.
 8. A method of fabricatinga membrane structure according to claim 1, wherein the wafer furtherincludes an etch-stop layer underlying the membrane layer, and whereinthe method further comprises the steps of: a) etching through the waferto partially form the window; and b) etching through the etch-stop layerto the membrane layer to fully form the window.
 9. The method offabricating a membrane structure according to claim 8, furthercomprising the step of mounting the membrane structure in a protectivefixture prior to the step of etching through the etch stop layer, inorder to protect the membrane layer.
 10. The method of fabricating amembrane structure according to claim 8, wherein the etch stop layer isremoved with a buffered oxide etch.
 11. A method of fabricating amembrane structure according to claim 1, wherein the materials of thewafer body, backside layer, the etch stop layer, and the membrane layerare selected so that when a window is formed through the wafer up to themembrane layer, the materials therein exhibit hoop stresses sufficientto provide tension to the membrane layer, to maintain the membrane layersubstantially flat.
 12. A method of fabricating a membrane structureaccording to claim 1, wherein the wafer body is formed of silicon, thebackside layer is formed of silicon nitride, the etch stop layer isformed of silicon dioxide, and the membrane layer is formed of silicon.13. A method of fabricating a membrane structure according to claim 1,further comprising the step of confirming that the flatness of themembrane structure is within allowable limits.
 14. A method offabricating a membrane structure according to claim 1, furthercomprising the step of baking the membrane structure for a length oftime at a temperature and humidity sufficient to promote native oxidegrowth on the membrane.
 15. A diffractive phased array assemblycomprising first and second membrane structures, each membrane structurehaving a membrane layer with a patterned grating defined thereon,wherein the membrane layers of the two membrane structures are inoperative contact with one another so as to form a gap therebetween. 16.A diffractive phased array assembly as recited in claim 15, wherein anindex matching fluid is provided within the gap formed between thepatterned surfaces of the first and second membrane structures.
 17. Adiffractive phased array assembly as recited in claim 16, wherein theindex matching fluid has an ultra low viscosity.
 18. A diffractivephased array assembly as recited in claim 15, wherein the first andsecond membrane structures are adapted for relative translationalmovement, substantially parallel to the membrane surface.
 19. Adiffractive phased array assembly as recited in claim 15, wherein eachof the first and second membrane structures is provided with internalstresses to apply tension to the membrane layer to maintain the membranelayer flat.
 20. A diffractive phased array assembly comprising: a platestructure having a patterned grating defined thereon and a membranestructure having a patterned grating defined thereon, wherein thepatterned grating of the plate and the patterned grating of the membraneare in operative contact with one another so as to form a gaptherebetween.
 21. A diffractive phased array assembly as recited inclaim 20, wherein an index matching fluid is provided within the gapformed between the patterned surfaces of the plate structure andmembrane structure.
 22. A diffractive phased array assembly as recitedin claim 20, wherein the plate structure and membrane structure areadapted for relative translational movement, substantially parallel tothe respective gratings.